Great question — answering as the UphillTech team:
The generic plastic bottle cap can be sealed outside of a factory. If you ever bought craft beer on tap take away, they will pour it in a plastic bottle and then seal it with a plastic cap. Then it looks completely legit. Plastic cap can be put on a bottle together with the plastic ring which holds it fast. But then it will hold the bottle with plastic spikes and will not go off without breaking ring apart from the cap. So it's totally unsurprising that somebody could easily replenish empty water bottles.
Full answer by alamar: https://travel.stackexchange.com/a/162417 (CC BY-SA 4.0).
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